1. Field of the Invention
This invention relates generally to magnetoresistive (MR) sensors, such as MR read heads for magnetic recording disk drives, and more particularly to MR sensors based on the spin accumulation effect.
2. Description of the Related Art
One type of conventional MR sensor, often called a “spin-valve” (SV) sensor, has a stack of layers that include two ferromagnetic layers separated by a nonmagnetic spacer layer. One ferromagnetic layer has its magnetization direction fixed, such as by being pinned by exchange coupling with an adjacent antiferromagnetic layer, and the other ferromagnetic layer has its magnetization direction “free” to rotate in the presence of an external magnetic field. With a sense current applied to the sensor, the rotation of the free-layer magnetization relative to the fixed-layer magnetization is detectable as a change in electrical resistance. The SV MR read head used in all current magnetic recording hard disk drives operates with the sense current directed parallel to the planes of the layers in the sensor layer stack, so it is referred to as a current-in-the-plane (CIP) sensor. In a disk drive CIP-SV read sensor or head, the magnetization of the fixed or pinned layer is generally perpendicular to the plane of the disk, and the magnetization of the free layer is generally parallel to the plane of the disk in the absence of an external magnetic field. When exposed to an external magnetic field from the recorded data on the disk, the free-layer magnetization will rotate, causing a change in electrical resistance.
A type of MR read head based on a magnetic tunnel junction (MTJ) has been proposed. In an MTJ MR read head the nonmagnetic spacer layer between the free and fixed ferromagnetic layers is a nonmagnetic tunnel barrier layer, typically formed of alumina. The MTJ MR read head operates with tunneling current perpendicular to the planes (CPP) of the layers in the sensor stack, and the tunneling current that passes perpendicularly through the layers depends on the relative orientation of the magnetizations in the two ferromagnetic layers.
A type of MR sensor and read head based on “spin accumulation” has been proposed in US 2005/0002128 A1 and US 2004/0257714 A1. This type of sensor is based on the experimental results published by F. J. Jedema et al., “Electrical detection of spin precession in a metallic mesoscopic spin valve”, Nature, Vol. 416, April 2002, pp. 713-716. This type of MR sensor has an electrically conductive strip with a first tunnel barrier and a free ferromagnetic layer on the front or sensing end of the strip and second tunnel barrier and a fixed ferromagnetic layer on the back end of the strip. When current is passed from the fixed ferromagnetic layer through the second tunnel barrier into the back end of the strip and the fixed ferromagnetic layer is magnetized in a direction so that the number of spin up electrons in that layer is higher than the number of spin-down electrons, then spin-up electrons accumulate below the second tunnel barrier. However since the strip is nonmagnetic the degree of the spin-accumulation decays exponentially with a characteristic length scale equal to the spin diffusion length of the of the electrons in the strip material. If the front end of the strip is located at a distance about equal to or shorter than the spin diffusion length then the spin accumulation is detected as a voltage across the front end. This voltage depends on the direction of magnetization of the free ferromagnetic layer so that when the free layer is exposed to an external magnetic field the electrical resistance across the first tunnel barrier changes.
For maximum read-head stability and output-linearity without hysteresis in the MR read head based on the spin accumulation effect, the magnetization of the free layer should be maintained in a saturated single domain state in the absence of an external magnetic field. In such a state, the local magnetization everywhere in the free layer, including the ends or side edges, is essentially “longitudinal”, i.e., along the length of the free layer and the cross-track direction of the head and parallel to the plane of the magnetic recording medium. The previously-cited references propose longitudinal biasing of the free layer by either ferromagnetic biasing layers located on opposite sides of the free layer or a ferromagnetic biasing layer located in the sensor stack and magnetostatically coupled with the free layer across a nonmagnetic spacer layer. The proposed in-stack biasing approach requires a second antiferromagnetic layer in the sensor to exchange-couple the biasing layer and thus a second annealing step in the presence of an applied field to set the magnetization direction of the biasing layer.
What is needed is an MR sensor based on the spin accumulation effect with improved in-stack biasing of the sensor free ferromagnetic layer.